I always wondered what would have happened if Angel turned a human into a vampire while he had a soul. It was interesting that a part of his soul seemed to remain in Lawson. Angel also said Lawson was the only one he sired while having a soul.

I also thought Spike looked great with black hair.

I have to admit, I'd wondered about what would happen if Angel sired a vampire while he had a soul as well...

He wasn't a vicious, cruel psychopath like all of them... just completely desensitized.

"It was interesting that a part of his soul seemed to remain in Lawson."

I am not sure we can conclude that this really happened. All vampires are different, though all the ones without souls are evil. Note that Spike told Angel that he never really considered his victims, and Angel responded that for him it was all about the evil. Lawson carried over his deep concern with purpose, and couldn't find one. His idea that Angel gave him part of his soul is just poetic, in my view. There can't really be "part" of a soul.
A bigger concern is that the ensouled Angel is responsible, in part, for the hundreds or thousands of lives taken or destroyed by Lawson. Was saving the sub worth it? I realize that it would have been "jerky" of Angel to destroy Lawson as soon as his purpose had been served, but in the long run, better. I felt the same when Cordelia spared Harmony, when Buffy spared Angel, and in some other cases. Charles Gunn destroyed his own sister for gosh sake!